Quantitative ultra-low temperature autoradiography, ultra-low temperature microdissection and the fluorescent microscopy of protein-fluorescein conjegates, are being used to study the permeability of the nuclear envelope, and the nuclear-cytoplasmic distribution in the intact amphibian oocyte and Chironomid salivary gland cell, of solutes of varying molecular sizes and chemical properties. The same techniques are being used to study the nuclear-cytoplasmic transport of amino acids and to establish the mechanisms responsible for non-uniform amino acid pools.